Sun burned Hosta before and after pruning:
Exposure to the intense sunlight of bright, cloudless, summer days can be too much for sensitive plants. Reflected light from walls and other surfaces can also add to the problem. Stunted plant growth and a yellow-white "burn" on the upper surface of older leaves are familiar symptoms of too much intense sunlight.
Hot air, particularly hot, dry wind, causes too much moisture loss from the plant's foliage. Some evaporation from leaves is normal, but when vital moisture is being evaporated faster than the plant's ability to replace it, leaves dry out and wilt. To be drought-tolerant, plants must have roots able to absorb as much, or more, moisture from the soil and do it as fast, or faster, than the foliage loses it. First symptoms of hot air injury are drying and browning at the tips and edges of older leaves. Then, tender new tip growth wilts, soon followed by dieback. Rapid moisture loss can cause tender leaves to turn black. Evaporation cools foliage, but if it doesn't get water from the roots fast enough to provide the evaporative cooling effect, the foliage gets hot, tender growth wilts and older leaves sunburn.
Hot, dry summers are rough on plants, especially on non-native plants and those weak from improper care. Since many of our landscape plants aren't naturally adapted to heat, they need special attention and care.
High temperatures speed up the normal living process of plants to a maximum rate at and above 90 degrees F. This means that most plants can take temperatures up to 90 degrees F. fairly well. Anything above that—the hotter it gets, the more they suffer! Of course, less tolerant or weaker plants suffer even more. The longer high temperatures persist, the greater the injury to the plant.
During the extreme hot weather remember too supply adequate water and fertilizer to potted plants & choosing the right location to protect them from the sun.